Tom is seven. He goes to school every day. The school is near his home. So he goes there on foot and comes back home on time (准时). But today he is late. His mother asks him, "Why are you late today?" "I was in the teachers' office.'' "Why did you go to the teachers' office?" "Because my teacher asked us a question in class, and nobody(没有人)could answer it, but me." "It's good to answer the teacher's questions." "But the question was 'Who put the ink (墨水) on my chair?'"